1. Field of the Invention
This invention relates to a semiconductor device having an electrostatic destruction protection circuit which prevents flow of excessively large current caused by a surge or the like and more particularly to a semiconductor device having an electrostatic destruction protection circuit which uses a thyristor (SCR) as a protection element.
2. Description of the Related Art
In a semiconductor device such as an IC and LSI, an electrostatic destruction (ESD) protection circuit is provided to protect an internal circuit from excessively large current caused by a surge or the like.
Various configurations of the electrostatic destruction protection circuit are proposed and circuits which protect the internal circuits by use of diodes and MOS transistors are widely used. However, as the integration density of the semiconductor device is more enhanced and the operation voltage is further lowered, it becomes impossible to sufficiently protect the internal circuit only by use of the diodes or MOS transistors. For example, in Jpn. Pat. Appln. KOKAI Publication No. 2003-318265 and International Patent Publication No. WO01/011685, an electrostatic destruction protection circuit which uses a thyristor as a protection element is proposed. The thyristor can perform the high-speed switching operation, permits large current to flow therein and is difficult to be broken. Therefore, the electrostatic destruction protection circuit using the thyristor has an excellent characteristic of high performance and high protection capability.
The electrostatic destruction protection circuit is configured by a thyristor, a plurality of diodes and a resistor. The anode of the thyristor is connected to a terminal (power supply terminal) to which power supply voltage is applied and the cathode thereof is connected to a terminal (ground terminal) to which ground potential is applied. The cathodes of the plurality of diodes are respectively connected to the anodes of the next-stage diodes and the plurality of diodes are connected between the power supply terminal and the gate of the thyristor. The diodes function as a trigger circuit of the thyristor. A resistor which sets the trigger voltage of the thyristor in cooperation with the plurality of diodes is connected between the gate of the thyristor and the ground terminal.
With the above configuration, if voltage between the power supply terminal and the ground terminal varies due to a surge or the like to a greater extent, current flows from the power supply terminal to the ground terminal via a plurality of diodes and resistor. Therefore, voltage occurs at the gate of the thyristor and trigger current is supplied to the gate of the thyristor based on the above voltage (trigger voltage). As a result, the thyristor is fired or triggered and short-circuits a path between the power supply terminal and the ground terminal to leakage the surge and protect the internal circuit. The trigger voltage is determined by the sum of the forward voltages VF of the plurality of diodes and the resistance of the resistor.
By further enhancing the integration density of the recent semiconductor device and lowering the operation voltage thereof, MOS transistors configuring the internal circuit are further miniaturized and the gate oxide film is made thinner and is liable to be destroyed. In order to safely protect the thus miniaturized MOS transistor, it is necessary to lower the trigger voltage which turns on the thyristor at the time of application of a surge voltage and set the trigger voltage lower than the breakdown voltage of the gate oxide film.
If the gate oxide film of the MOS transistor is thick, voltage applied between the power supply terminal and the ground terminal exceeds the maximum value of the power supply voltage and the thyristor is turned on in a range of voltage lower than the breakdown voltage of the gate oxide film. Therefore, the internal circuit can be effectively protected from the excessively large current caused by a surge. However, if the gate oxide film is thin, the breakdown voltage of the gate oxide film is lowered and a difference between the maximum value of the power supply voltage and the breakdown voltage of the gate oxide film becomes smaller. As a result, there occurs a possibility that voltage which turns on the thyristor may exceed the breakdown voltage of the gate oxide film. That is, the gate oxide film of the MOS transistor configuring the internal circuit will be destroyed before the protection operation by the electrostatic destruction protection circuit is performed.
In order to realize the low trigger voltage in the electrostatic destruction protection circuit using the thyristor as described above, it is necessary to reduce the number of stages of the diodes which function as the trigger circuit. However, if the number of stages of the diodes is reduced, the leakage current flowing from the power supply terminal to the ground terminal through the diodes and resistor in the normal operation mode becomes large and the current consumption increases.